The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Many applications in neuroscience and neural prosthetics would benefit from having three-dimensional arrays of electrodes to allow the simultaneous monitoring of interactions among networks of neurons spanning multiple layers of brain. However, creating practical three-dimensional arrays has remained a challenge. Typically, neural probes are batch fabricated using planar processing techniques, resulting in two-dimensional electrode configurations which have to be micro-assembled to form 3-D arrays. The existing assembly approaches are tedious and result in fragile and oversized devices. The present teachings present a novel approach to 3-D microelectrode array formation and wire overlay that enables easy high-yield assembly and pushes the limits of miniaturization.
Several approaches exist for electrically interfacing with neurons in a volume of tissue. The earliest, cheapest and most widely available method involves microwire arrays which are typically bundled together with tips staggered at different heights. A silicon-based alternative to the microwire solution was developed at the University of Utah. Although the physical structure of these arrays appears three-dimensional, neither can be considered true 3-D electrical interfaces because they lack multiple channels that simultaneously span the longitudinal, transverse and vertical dimensions. Two-dimensional arrays fabricated back-to-back that fold into 3-D arrays have also been demonstrated, but these are inherently limited to only two parallel sets of shanks.
True 3-D interfaces formed by assembling 2-D arrays in parallel have been demonstrated in the past. However, the assembly methods developed thus far are tedious, preventing the 3-D arrays from being supplied in quantity. The past approaches to assembling two-dimensional probes (passive or active) involve inserting the individual shanks 102 on each probe into corresponding holes formed in a thin silicon platform 104 and securing the multiple probes in parallel with orthogonally-fitted comb-like structures, called spacers, as shown in FIGS. 1(a)-(c) and 2(a)-(b). In this assembly process, the first step is to orthogonally bend the gold tabs on each probe wing such that they are parallel to the platform surface. In this state, the probe back end is held by a vacuum pick that is connected to a 3-way micromanipulator. Then the shanks are orthogonally aligned to the holes in the platform and dropped into the platform. This process is repeated for each probe making up the 3-D array. Next, the silicon spacer 106 is fitted and used to stabilize all probes, which otherwise would wobble due to the weight of the protruding back end 108. Finally with the probes in place and stabilized on the platform, the gold tabs on each probe wing 110 are ultrasonically bonded to the platform. A picture of an assembled 3-D array using four parallel active probes orthogonally assembled on a silicon platform and stabilized by silicon spacers 106 is shown in FIGS. 2(a)-(c).
This approach has a number of disadvantages. The 2-D arrays used for 3-D assembly are specifically designed with lateral wings that take significant space, not only from the device point of view but also on the mask. The thin silicon platform (˜15 μm), defined by a boron etch-stop process, must carry the assembled probes and perhaps other integrated circuit components, and while it is supported on a solid metal block during assembly, it is fragile and difficult to use for multiple implants. The idea of individual holes in the platform for each shank has merit for encapsulation around each shank as was demonstrated with a glass frit reflow process, but results in a tedious assembly procedure since each shank must be precisely aligned before the entire probe can be inserted. Once all probes are inserted into the platform, they must be manually held in parallel relation while the spacer is being aligned and fitted. This is yet another tedious and time consuming step. In bonding the lead tabs, the bond wedge (typically 100 μm at the tip, tapering at 15°) must be able to access tabs in between the wings, limiting the array spacing. The bond wedge must also access the inner-most tab on each wing without interfering with the back end, which results in “dead” space on the wing that places the inner-most tab a minimum distance away from the back end of the probe. A substantial vertical rise of the array above the platform cannot be avoided even with passive probes since vertical spacers are used for stabilization. This is a major limitation, especially with active probes, that complicates or even prohibits the post-implant procedure of replacing the dura over the device. Although a folding back end technology was developed, the vertical rise is still of concern since multiple back ends are stacked on top of each other. Furthermore, the folding technique is not effortless. The successful assembly of just one 3-D array using the described approach can take an hour or more. Even then, these structures remain relatively large and fragile for fully implantable applications.